Structured metal parts made of titanium or titanium alloys are conventionally made by casting, forging or machining from a billet. These techniques have a disadvantage of high material waste of the expensive titanium metal and large lead times in the fabrication of the metal part.
Fully dense physical objects may be made by a manufacturing technology known as rapid prototyping, rapid manufacturing, layered manufacturing, solid freeform fabrication, additive fabrication, additive manufacturing or 3D printing. This technique employs computer aided design software (CAD) to first construct a virtual model of the object which is to be made, and then transform the virtual model into thin parallel slices or layers, usually horizontally oriented. The physical object may then be made by laying down successive layers of raw material in the form of liquid, paste, powder or other layerable, spreadable or fluid form, such as melted metal, e.g., from a melted welding wire, or preformed as sheet material resembling the shape of the virtual layers until the entire object is formed. The layers are fused together to form a solid dense object.
Solid freeform fabrication is a flexible technique allowing creation of objects of almost any shape at relatively fast production rates, typically varying from some hours to several days for each object. The technique is thus suited for formation of prototypes and small production series, and can be scaled-up for large volume production.
The technique of layered manufacturing may be expanded to include deposition of pieces of the construction material, that is, each structural layer of the virtual model of the object is divided into a set of pieces which when laid side by side form the layer. This allows forming metallic objects by welding a wire onto a substrate in successive stripes forming each layer according to the virtual layered model of the object, and repeating the process for each layer until the entire physical object is formed. The accuracy of the welding technique is usually too coarse to allow directly forming the object with acceptable dimensions. The formed object will thus usually be considered a green object or pre-form which needs to be machined to acceptable dimensional accuracy.
It is known to use a plasma arc to provide the heat for welding metallic materials. This method may be employed at atmospheric or higher pressures, and thus allow simpler and less costly process equipment. One such method is known as gas tungsten arc welding (GTAW, also denoted as TIG) where a plasma transferred arc is formed between a non-consumable tungsten electrode and the welding area. The plasma arc is usually protected by a gas being fed through the plasma torch forming a protective gas shield around the arc. TIG welding may include feeding a metal wire or metal powder into the melting pool or the plasma arc as a filler material. Other welding methods include gas metal arc welding (GMAW), metal inert gas (MIG) welding and metal active gas (MAG) welding, were an electric arc between a consumable electrode, such as a metal wire, and the workpiece heats and melts the metal.
It is known (e.g., see Adams, U.S. Pat. Pub. No. 2010/0193480) to use a TIG-welding torch to build objects by solid freeform fabrication (SFFF), where successive layers of metallic feedstock material with low ductility are deposited onto a substrate. A plasma arc is created by energizing a flowing gas using an electrode, the electrode having a variable magnitude electric current supplied thereto. The plasma stream can be directed towards a predetermined targeted region to preheat the predetermined targeted region of the workpiece prior to deposition. The current is adjusted and the feedstock material is fed into the plasma stream to deposit molten feedstock in the predetermined targeted region. The electric current is adjusted and the molten feedstock is slowly cooled at an elevated temperature, typically above the brittle to ductile transition temperature of the feedstock material, in a cooling phase to minimize the occurrence of material stresses.
Withers et al. (U.S. Pat. Pub. No. 2006/185473) also describes using a TIG torch in place of the expensive laser traditionally used in a solid freeform fabrication (SFFF) process with relatively low cost titanium feed material by combining the titanium feed and alloying components in a way that considerably reduces the cost of the raw materials. More particularly, in one aspect the present invention employs pure titanium wire (CP Ti) which is lower in cost than alloyed wire, and combines the CP Ti wire with powdered alloying components in-situ in the SFFF process by combining the CP Ti wire and the powder alloying components in the melt of the welding torch or other high power energy beam. In another embodiment, the invention employs titanium sponge material mixed with alloying elements and formed into a wire where it may be used in an SFFF process in combination with a plasma welding torch or other high power energy beam to produce near net shaped titanium components.
In order to effectively deposit metal from a metal wire onto the surface of a work piece using a welding torch, it is necessary to maintain the metal wire in the correct position relative to the welding torch. Metal wire often is provided off of a spool. The output torque of motors driving rotation of the wire spool can be a limiting factor in providing wire at a steady state, particularly from a fully loaded spool. The rotational inertia of the wire on the spool can limit the speed and acceleration rate at which the wire can be unwound off of the spool to be delivered to the welding torch. Modulation in rotational inertia, speed and/or acceleration can be sufficient to cause slippage from the rollers, guiding wheels or clamping devices used to deliver the wire to the plasma arc of the contact tip assembly. Slippage can result in deformation of the wire, and also creating a deviation in the desired position and angle of the wire relative to the plasma arc. Slippage also limits the operating speed of the fabricating equipment.
Changes in rotational mass, speed and/or acceleration of the bulk wire also can result in variation in the wire feed speed and variations in the amount of tension in the metal wire. If the variation in wire feed speed or acceleration of the wire at the wire source results in too much tension between the wire source and the feeder rollers and pulleys that deliver the wire to the plasma arc of the welding torch, the increased tension can result in the formation a kink, bend or other deformation in the wire. High tension also can result in metal wire being pulled back toward the wire source, which would prevent feed of metal wire to the welding electrode, resulting in an unwanted discontinuous deposition layer, or an unintended hole or gap in the layer being deposited on the freeform object being made. If the tension is too low, an excess of slack wire can result. The excess wire can become entangled with itself or a part of the machinery, which can cause bends or kinks in the wire, making it difficult or impossible to correctly position the wire relative to the plasma arc.
In addition, unwinding of the wire from the spool results in changes in the position of the wire as it leaves the spool due to the coiled nature of the wire on the spool. At higher rates of use, the horizontal and vertical position of the wire rapidly can change, which can result in slippage from the rollers, guiding wheels or clamping devices used to deliver the wire to the welding torch.
Accordingly, there exists a need in this art for an economical method of performing freeform fabrication at an increased rate of metal deposition. Furthermore, there exists a need in this art for a system and method of increasing the amount of metal wire that can be provided to a welding torch without slippage or deformation of the metal wire in order to increase the throughput and yield of direct metal deposition formed products.